gltf rendering

[Walther]

Initial support for rendering gltf scenes.

Fixes #151

In scope

• [x] Reading a .gltf file
• [x] Triangle primitive
• [x] Base color texture
• [x] Metallic-Roughness texture
• [x] Normal map texture
• [x] Emissive texture
• [x] Occlusion texture

Out of scope

• Specification-accurate metalness-roughness model, improve later
• Cameras provided in the .gltf file
• Non-triangle primitives
• Computing tangents and normals if not given in the file
• Transparency in textures, blending modes
• Skinning
• Animations
• Optional extensions